agent-skills/skills/vercel-react-native-skills/rules/js-hoist-intl.md

title, impact, impactDescription, tags

title	impact	impactDescription	tags
Hoist Intl Formatter Creation	LOW-MEDIUM	avoids expensive object recreation	javascript, intl, optimization, memoization

Hoist Intl Formatter Creation

Don't create Intl.DateTimeFormat, Intl.NumberFormat, or Intl.RelativeTimeFormat inside render or loops. These are expensive to instantiate. Hoist to module scope when the locale/options are static.

Incorrect (new formatter every render):

function Price({ amount }: { amount: number }) {
  const formatter = new Intl.NumberFormat('en-US', {
    style: 'currency',
    currency: 'USD',
  })
  return <Text>{formatter.format(amount)}</Text>
}

Correct (hoisted to module scope):

const currencyFormatter = new Intl.NumberFormat('en-US', {
  style: 'currency',
  currency: 'USD',
})

function Price({ amount }: { amount: number }) {
  return <Text>{currencyFormatter.format(amount)}</Text>
}

For dynamic locales, memoize:

const dateFormatter = useMemo(
  () => new Intl.DateTimeFormat(locale, { dateStyle: 'medium' }),
  [locale]
)

Common formatters to hoist:

// Module-level formatters
const dateFormatter = new Intl.DateTimeFormat('en-US', { dateStyle: 'medium' })
const timeFormatter = new Intl.DateTimeFormat('en-US', { timeStyle: 'short' })
const percentFormatter = new Intl.NumberFormat('en-US', { style: 'percent' })
const relativeFormatter = new Intl.RelativeTimeFormat('en-US', {
  numeric: 'auto',
})

Creating Intl objects is significantly more expensive than RegExp or plain objects—each instantiation parses locale data and builds internal lookup tables.